1 Technical Field of the Invention
The present invention relates generally to an FM-CW radar apparatus which may be employed in anti-collision systems installed in moving objects such as automotive vehicles and which is designed to transmit a frequency-modulated radar wave and receive a radar wave reflected from a target object to determine the distance to and relative speed of the target object.
2 Background Art
Recently, a radar is tried to be used in an anti-collision device of automotive vehicles. FM-CW (frequency-modulated continuous wave) radars designed to measure both the distance to and relative speed of a target are proposed for ease of miniaturization and reduction in manufacturing cost thereof.
Typical FM-CW radars transmit a radar signal Ss, as indicated by a solid line in FIG. 12(a), which is frequency-modulated with a triangular wave to have a frequency increased and decreased cyclically in a linear fashion and receive a radar return of the transmitted radar wave from a target. The received signal Sr, as indicated by a broken line, undergoes a delay of time Td the radar signal takes to travel from the radar to the target and back, that is, time depending upon the distance to the target, so that the received signal Sr is doppler-shifted in frequency by Fd depending upon the relative speed of the target. The received signal Sr and the transmitted signal Ss are mixed together by a mixer to produce a beat signal Sb, as shown in FIG. 12(b), whose frequency is equal to a difference in frequency between the received signal Sr and the transmitted signal Sb. If the frequency of the beat signal Sb when the frequency of the transmitted signal Ss is increased, which will be referred to below as a beat frequency in a modulated frequency-rising range, is defined as fu, the frequency of the beat signal Sb when the frequency of the transmitted signal Ss is decreased, which will be referred to below as a beat frequency in a modulated frequency-falling range, is defined as fd, then distance R to and relative speed V of the target may be expressed as: ##EQU1## where c is the propagation speed of a radio wave, T is a cycle of the triangular wave for modulation of the transmitted signal Ss, .DELTA.F is a variation in frequency of the transmitted signal Ss, and Fo is a central frequency of the transmitted signal Ss.
In order to use such an FM-CW radar in automotive vehicles, it is necessary to design the FM-CW radar so that it can detect a target located a maximum of 100 to 200 meters away in a resolution in range of at least several meters.
The resolution in range of the FM-CW radar may be expressed by the equation (3) below. ##EQU2##
The equation (3) shows that establishment of the resolution in range of several meters requires the frequency variation .DELTA.F of the order of 100 MHz. Assuring such a frequency variation requires setting the central frequency of the transmitted signal Ss to within a frequency band of several tens GHz to several hundreds GHz (i.e., a millimeter wave).
For instance, when the frequency variation .DELTA.F is 100 MHz, and the cycle T is 1 ms and when the relative speed V of the target is zero (i.e., fu=fd), and the distance R to a target object is 100 m, the beat frequencies fu and fd will be 133 KHz. When the target object is present within 100 m, the beat signal Sb of less than 133 KHz is produced. When the relative speed V is not zero (0), the beat signal Sb is produced which has a doppler-shifted frequency increasing and decreasing across the frequency when the relative speed V is zero (0). Specifically, the use of the FM-CW radar in automotive vehicles requires the ability to produce a beat signal of several tens KHz to several hundreds KHz.
Usually, in high-frequency mixers handing a signal in a high frequency band such as a millimeter wave, AM-FM conversion-caused noises including frequency components of fluctuation in signal strength and/or 1/f noises having the strength inversely proportional to the frequency are superimposed on a mixer output. The AM-FM conversion-caused noises and the 1/f noises, which will be referred to as low frequency noises below, are relatively great in strength in the same frequency band as that of the beat signal Sb (i.e., several tens KHz to several hundreds KHz), thus resulting in a deterioration in signal-to-noise (SN) ratio of the beat signal Sb.
Japanese Patent First Publication No. 5-40169 discloses, as shown in FIG. 13, an FM-CW radar 110 consisting of a high-frequency oscillator 112, a modulating signal generator 126, a transmitting antenna 116, a receiving antenna 120, a distributor 118, a high-frequency mixer 122. The high-frequency oscillator 112 produces a high-frequency output signal Ss. The modulating signal generator 126 generates a modulating signal for modulating the frequency of the output signal Ss so as to take the form of a triangular wave. The transmitting antenna 116 transmits the output signal Ss in the form of a radar wave. The receiving antenna 120 receives a radar return of the transmitted radar wave from a target object to provide an input signal Sr to the high-frequency mixer 122. The high-frequency mixer 122 mixes a local signal L from the distributor 118 with the input signal Sr from the receiving antenna 120 to produce a beat signal Sb. The FM-CW radar also includes a second oscillator 136, a switching circuit 138, band-pass filters 132 and 140, and an intermediate-frequency mixer 134. The second oscillator 136 produces a switching signal with a frequency more than two times that of the beat signal Sb. The switching circuit 138 is responsive to the switching signal from the second oscillator 136 to permit the input signal Sr from the receiving antenna 120 to be supplied to the high-frequency mixer 122 cyclically. The high-frequency mixer 122 mixes the local signal L with the input signal Sr from the receiving antenna 120 to produce the beat signal Sb. The band-pass filter 132 passes only a frequency component of the beat signal Sb produced in a frequency domain corresponding to the frequency of the switching signal. The intermediate-frequency mixer 134 mixes the frequency component transmitted through the band-pass filter 132 with the switching signal which is produced by the second oscillator 136 and shaped by the band-pass filter 140 to have the beat signal Sb fall into a desired frequency band of several tens KHz to several hundreds KHz.
The frequency Fr of the input signal Sr received by the receiving antenna 120 at time t is shifted from the frequency Ft of the output signal Ss transmitted at time t by the beat frequency fu which depends upon the distance to and relative speed of the target object reflecting the radar wave. The frequency spectrum of the input signal Sr outputted from the switching circuit 138, thus, has sidebands, as shown in FIG. 14(a), shifted from the central frequency Fr by the frequency Fs of the switching signal outputted from the second oscillator 136, respectively. The input signal Sr is, as described above, mixed with the local signal L by the high-frequency mixer 122 to produce the beat signals Sb having a frequency component corresponding to a difference in frequency between the signals Sr and L.
FIG. 14(b) shows the frequency spectrum of the input signal Sr (solid line) and the frequency spectrum of the local signal L (broken line).
The frequency spectrum of the beat signal Sb produced by the high-frequency mixer 122, as can be seen from the drawing, has signal components with the beat frequency fu (=.vertline.Fr-Ft.vertline.) corresponding to a difference between the central frequency of the input signal Sr and the frequency of the local signal L and with the frequencies Fs.+-.fu that correspond to the differences in frequency between sideband components of the input signal Sr and the local signal L. These signal components are shown in FIG. 15.
In the following discussion, one of components of the beat signal Sb which has the beat frequency fu will be referred to as a fundamental component, and the other components having the frequencies Fs.+-.fu will be referred to as harmonic components.
In the FM-CW radar 110, production of the beat signal Sb through the high-frequency mixer 122 which has the harmonic components within a frequency domain of a few MHz almost insensitive to lower frequency noises is achieved by setting the frequency of the switching signal to a few MHz. The intermediate-frequency mixer 134 which adjusts the harmonic components to fall within a desired frequency band may handle signals much lower in frequency than the millimeter waves and is smaller in lower frequency noises superimposed on an output thereof than the high-frequency mixer 122, thus resulting in an improved SN ratio of the beat signal Sb.
It is, however, found that the switching circuit 138 is subjected to significant changes in input/output impedance in on- and off-states, which adversely affects the activities of the high-frequency mixer 122 connected to the switching circuit 138. Specifically, since the high-frequency mixer 122 is usually so adjusted as to provide a matched input impedance when the switching circuit 138 is in the on-state, the impedance matching is not established when the switching circuit 138 is in the off-state, thereby resulting in instability of an operation of the high-frequency mixer 122, which may cause the high-frequency mixer 122 to be oscillated.
In order to avoid the above problem, the publication No. 5-40169 teaches interposing an isolator (circulator) between the switching circuit 138 and the high-frequency mixer 122 or use of a low noise amplifier as the switching circuit 138 which functions as an isolator.
The drawback is, however, encountered in that the isolator dampens the input signal Sr to the high-frequency mixer 122, thus resulting in deterioration in radar sensitivity. Further, when the lower noise amplifier is used as the switching circuit 138, it does not dampen the input signal Sr as much as by the isolator, but an isolation effect is not obtained which is enough to eliminate the influence of variation in impedance of the switching circuit 138 on the high-frequency mixer 122.
The above publication also teaches interposing a switching circuit 138 between the high-frequency oscillator 112 and the transmitting antenna 116, but it is necessary to install an isolator between the high-frequency oscillator 112 and the switching circuit 138 in order to eliminate the influence of variation in impedance of the switching circuit 138 on the high-frequency mixer 122, which will lead to the same problem encountered in the structure in which the switching circuit 138 is disposed between the receiving antenna 120 and the high-frequency mixer 122.